Thread-severing device for sewing machine

The invention provides a thread-severing device for a sewing machine, which has no use for the adjustment operation of a synchronizator even if the thread is changed, can prevent poor thread, and lighten the burthen of a user. The sewing machine has a sewing machine motor, a pedal, a sewing needle position detecting unit, a thread-severing unit and a control device, wherein the thread-severing device has a thread thickness detecting unit for detecting the thickness of the thread and outputting the signal of the thread thickness, and a storage cell for storing a driving pulse number transmitted to the sewing machine motor from the stop of a driving unit to the start until the brake signal is transmitted to the sewing machine motor corresponding to the thickness of each thread, the control unit reads the driving pulse number from the storage cell according to the thread thickness signal received from the thread thickness detecting unit, and transmits the brake signal to the sewing machine motor after transmitti